The present invention relates to a fiber optic gyro which employs a semiconductor laser or super luminescent diode as a light source and, more particularly, to a fiber optic gyro designed for less power consumption and wider application range of temperature.
FIG. 1 shows a conventional fiber optic gyro. Light emitted from a light source 11 is applied via an optical coupler 12 and a polarizer 13 to an optical coupler 14, from which it is input, as right-handed light and left-handed light, into an optical fiber coil 15 at both ends thereof. The right-handed and the left-handed light having propagated through the optical fiber coil 15 in opposite directions are coupled together by the optical coupler 14 and interfere with each other. The resulting interference light is provided via the polarizer 13 and the optical coupler 12 to a photodetector 16, wherein it is converted into an electrical signal.
When no angular velocity is being applied to the optical fiber coil 15 in the peripheral direction thereof, the phase difference between the right-handed and the left-handed light in the optical fiber coil 15 is substantially zero. When an angular velocity .OMEGA. is applied to the optical fiber coil 15 in its peripheral direction, a so-called Sagnac effect is produced, by which a phase difference is introduced between the right-handed and the left-handed light in the optical fiber coil 15. As is well-known in the art, the phase difference .DELTA..phi..sub..OMEGA. is expressed by ##EQU1## where R is the radius of the optical fiber coil 15, L is the length of the optical fiber coil 15, .lambda. is the wavelength of light in a vacuum, C is the velocity of light, and .OMEGA. is the input angular velocity.
The intensity I.sub.O of the interference light having reached the photodetector 16 is as follows: ##EQU2## where Imax is a maximum quantity of light.
Therefore, the angular velocity .OMEGA. can be detected by measuring the intensity I.sub.O of the interference light. In this instance, however, when the input angular velocity .OMEGA. is small, the phase difference .DELTA..phi..sub..OMEGA. is small and therefore a change in cos .DELTA..phi..sub..OMEGA. is small, so that the input sensitivity becomes extremely low.
To avoid this and hence optimize the input sensitivity, it is customary in the prior art to interpose between one end of the optical fiber coil 15 and the optical coupler 14 a phase modulator 17 in which the right-handed and the left-handed light are phase modulated by a drive signal available from a phase modulator drive circuit 18 as shown in FIG. 1. FIG. 2 shows the relationship between the phase modulating signal S.sub.M and the interference light when the signal S.sub.M is a sine wave, and FIG. 3 shows the abovesaid relationship when the signal S.sub.M is a rectangular wave.
In either case, the intensity I.sub.O of the interference light resulting from the phase modulation contains the following component: EQU Is=Ks.multidot.sin .DELTA..phi..multidot.cos (.omega.t+.theta.)(3)
where Ks is a constant, .omega. is the angular velocity of phase modulation, and .theta. is the phase difference between the phase modulating signal S.sub.M and the Ks.multidot.sin .DELTA..phi. component.
Synchronously detecting the output Signal of the photodetector 16 by a synchronous detector 19 with a signal which is synchronized with the phase modulating signal S.sub.M, the synchronous detector 19 yields the following output V: EQU V=K.multidot.sin .DELTA..phi. (4)
where K is a constant.
The phase .DELTA..phi. in Eq. (4) is the phase difference between the right-handed and the left-handed light in the optical fiber coil 15, which is expressed as follows: EQU .DELTA..phi.=.DELTA..phi..sub..OMEGA. +.DELTA..phi..sub.f ( 5)
where the phase difference .DELTA..phi..sub.f represents the phase difference introduced between the right-handed and the left-handed light by a feedback phase modulator 21 interposed between the other end of the optical fiber coil 15 and the optical coupler 14 as depicted in FIG. 1. The phase difference .DELTA..phi..sub.f can be created by phase shifting the right-handed and the left-handed light at a fixed rate by the feedback phase modulator 21. In practice, the phase shift is effected by applying a ramp voltage from a ramp voltage generator 22 to the feedback phase modulator 21.
When applying the ramp voltage to the feedback phase modulator 21, the right-handed light (hereinafter referred to as CW light) undergoes such a phase shift as indicated by the solid line at row A in FIG. 4, whereas the left-handed light (hereinafter referred to as CCW light) is delayed behind the CW light for a period of time .tau. for the propagation of the CCW light through the optical fiber coil 15 and is then similarly phase shifted as indicated by the broken line. As a result of this, the phase difference .DELTA..phi..sub.f between the CW light and the CCW light becomes such as shown at row B in FIG. 4. In this case, if the ramp voltage is applied so that a maximum value of the phase shift .phi..sub.R of each light is 2.pi.k (where k is an integer), the phase difference .DELTA..phi..sub.f between the CW light and the CCW light is expressed as follows: ##EQU3## where f is the frequency of the ramp voltage. The output of the synchronous detector 19 is applied to an integrating filter 23, the output of which is provided to the ramp voltage generator 22, thereby generating positive and negative ramp voltages of a frequency corresponding to that of positive and negative input voltages. By controlling the frequency of the ramp voltage so that the Sagnac phase difference .DELTA..phi..sub..OMEGA. between the CW light and the CCW light in the optical fiber coil 15 is cancelled by the phase difference .DELTA..phi..sub.f, a closed loop is established and the frequency f of the ramp voltage is obtained from Eqs. (1), (5) and (6) as follows: ##EQU4## By measuring the frequency f of the ramp voltage based on Eq. (7), the input angular velocity .OMEGA. can be obtained with 2R/n.lambda.k as a proportional constant. Incidentally, since it is easy for those skilled in the art to measure the frequency f of the ramp voltage, no particular means therefor is shown in FIG. 1.
The light source 11 may have such a structure as shown in FIGS. 5A and 5B which are horizontal and vertical sectional views. In a case 24 closed by a lid 25 a heat sink 26 for a light source element 27 is disposed, on which the light source element 27 is mounted, and a spherical end optical fiber 29 having a spherical tip is fixed by solder 31 on an optical fiber support 28. The light source element 27 and the spherical end optical fiber 29 are appropriately aligned so that light from the former is incident to the latter. On the heat sink 26 there are mounted a Peltier device 32 for cooling it and a thermistor 33 for sensing its temperature. Further, a photodetector 34 for monitoring the quantity of light is provided which receives light from the light source element 27. The light source element 27, the thermistor 33 and the photodetector 34 are connected to hermetic terminals 35 mounted on the case 24, which is, in turn, mounted on a mounting plate 36.
As depicted in FIG. 6, the output of the photodetector 34 is applied to an automatic light control circuit 37, by which the quantity of light emanating from the light source element 27 is controlled constant. The thermistor 33 is connected to a bridge circuit 39 in a temperature control circuit 38. The bridge circuit 39 yields an error signal between the temperature of the heat sink 26 detected by the thermistor 33 and a predetermined temperature. The error signal is amplified by an error signal amplifier 41 and is then provided to a Peltier device drive current generator 42, by which the drive current for the Peltier device 32 is controlled so that the temperature of the heat sink 26 becomes equal to the predetermined value.
As the light source element 27 for use in the light source 11, a super luminescent diode (hereinafter referred to simply as SLD), which emits light of a large spectral width, is often employed for the purpose of lessening the influence of noise arising from the backward Rayleigh scattering, coupling of light between different polarization modes and the Kerr effect in the optical fiber coil 15, that is, for the purpose of providing a highly accurate fiber optic gyro. However, the drive current of the SLD greatly varies with the temperature of the heat sink 26. FIG. 7 shows the relationship of the SLD drive current to the temperature of the heat sink 26 in an SLD module.
Now, assuming that the quantity of light emitted from the spherical end optical fiber 29 of the SLD module is set to one-half of a maximum quantity of light 1 mW available from the SLD, that is, 500 .mu.W, the temperature applicable to the SLD heat sink 26 is around 45.degree. C. at the highest; namely, the SLD module cannot be used in, for example, a +85.degree. C. atmosphere needed in MIL or the like.
In general, the temperature of the SLD heat sink 26 is controlled to remain constant, for instance, at 40.degree. C. by using the Peltier device 32. The Peltier device 32 used in the light source module shown in FIGS. 5A and 5B has such characteristics as shown in FIG. 8.
In FIG. 8, Q indicates the quantity of heat discharged on the cooling side of the Peltier device 32, which can be made to correspond to the quantity of heat generated by the light source element 27 mounted on the cooling surface. Strictly speaking, the amount of heat discharged on the cooling surface of the light source element 27 includes heat traveling from the spherical end optical fiber 29 and heat by radiation and convection in the light source module as well as heat generated by the light source element 27. Hence it must be taken into account that the actual performance of the Peltier device 23 is lower than its performance depicted in FIG. 8, obtained by testing the Peltier device singly. As seen from FIG. 8, when the Peltier device drive current exceeds 1 A, the heat absorption efficiency decreases. Therefore, the Peltier device drive current may preferably be 1 A or more.
Assuming that the output light of the spherical end optical fiber 29 is 500 .mu.W as mentioned previously, the surrounding temperature at which the Peltier device drive current exceeds 1 A is about 81.degree. C., below the required temperature +85.degree. C., as seen from data during continuous drive of the SLD shown in FIG. 9.
On the other hand, power that is consumed for temperature control of the SLD module is expressed as the sum of power consumed by the temperature control circuit 38 and power by the Peltier device 32. Since the power required for the bridge circuit 39 and the error signal amplifier 41 is smaller than maximum power for the Peltier device drive current generator 42 and the Peltier device 32, the overall consumed power is determined essentially by the power that is consumed by the Peltier device drive current generator 42 and the Peltier device 32. Now, assuming that the power source voltage of the Peltier device drive current generator 42 is a voltage 5 V which permits the Peltier device 42 to stably effect temperature control and is considered to be relatively easily available from the system employing the fiber optic gyro and that a maximum current flowing through the Peltier device 42 is limited to 1 A, maximum power consumed by the temperature control circuit 38 is 5 W. This value, considered in combination with consumed power of other electric circuits, is equal to or greater than consumed power of a conventional gyroscope which utilizes the Coriolis effect.
As described above, the conventional fiber optic gyro employs the SLD element for higher measuring accuracy and controls the temperature of the SLD element by the Peltier device for enlarging the working temperature range. Moreover, its light source module usually employs a small Peltier device for the purposes of minimizing its power consumption for temperature control and miniaturizing the light source module.
Therefore, the conventional fiber optic gyro is inoperable at the highest temperature in its working temperature range, and in spite of using the small Peltier device, power consumption is maximum power consumed as much as 5 W for temperature control alone, which is equal to or greater than the power consumption of the gyroscope utilizing the Coriolis effect.